Integrated circuit (IC) devices can be permanently damaged by excessive temperatures. Those IC devices that can generate sufficient self heating to cause such damage are commonly provided with internal means for shutting the circuit down when its internal temperature exceeds some limiting value. Typically, an IC element that has a temperature responsive characteristic, such as the forward voltage drop across a conducting PN junction diode, is monitored and a shutdown mechanism initiated when the temperature exceeds some predetermined level.
One such system, employing the monitoring of a sense emitter in an IC power transistor, is disclosed in a copending patent application by Robert J. Widlar, Ser. No. 773,693, filed Sept. 9, 1985. Here the power transistor emitter-base potential is compared with the sense emitter to base potential.
In U.S. Pat. No. 4,345,218 to Tim D. Isbell and James S. Congdon, issued Aug. 17, 1982, a thermal shutdown circuit is based upon an excessive temperature level being exceeded. After such an excess is sensed the IC is cycled between a lower high temperature state and a substantially lower low temperature state until the overload is cleared or the circuit is turned off. Thus, the highest temperature is encountered only once. In this circuit the temperature responsive element is a zener diode in combination with an emitter follower so that the main temperature responsive element is the base to emitter voltage of a conducting transistor.
The teaching in the above citations is incorporated herein by reference.